I dont think that neither M-Gengar nor Shadow tag should be banned, those 2 are not that Overpowered, in order to make M-Gengar go well, you need to make a switch-in against a Staller, but M-Gengar may have problems enduring hits including defensive Pokemon, M-Gengar is very fragil to take more than 3 or 4 hits of a defensive Pokemon. Other way to make M-Gengar´s appeareance is by sacrificing Pokemon of your team or making very good predictions sometimes, but if it were not for M-Gengar, GeoXern´s only counter may be Aegislash, and it is only a check, not a counter.
The Metagame is not overcentralizing all over M-Gengar, it is more centralized in Yveltal, Xerneas, Arceus, and one of the most: Ho-Oh... with the great buff of Defog, now Ho-Oh is almost in every game with almost all it´s variants, and M-Gengar have serious problems against this Pokemon, it has great Speed and Sp.Atk, but if that is the case, Deoxys-N and Deoxys-A are very much better than M-Gengar.
One thing is true, M-Gengar provokes fear, but the way to get rid of it, is to take it down even if your best non Scarfed Pokemon sharer it´s destiny with M-Gengar´s Destiny Bond.
M-Gengar is extremelly weak against Sticky Web, without Speed, M-Gengar is nothing but an amorphous purple shadow, it is the same if it faces a Scarfed Pokemon, M-Gengar have no opportunities against them because of its fragility.
It is like Deoxys-N but you cannot escape, that it all.
I vote NOT BAN M-GENGAR NOR SHADOW TAG.